1. Field of the Invention
The present invention relates to a picture processing system, and more particularly to reversible transform coding for interlaced scanned moving pictures.
2. Description of the Related Art
Various reversible coding systems for moving pictures are conventionally known, and one of reversible coding systems of the type mentioned has been proposed by the inventor of the invention of the present application (the reversible coding system is hereinafter referred to as prior art by the inventor). The prior art by the inventor is directed to a reversible coding system for pictures which is based on discrete cosine transform and allows reversible coding by modifying ordinary discrete cosine transform. A reversible moving picture coding system can be implemented by applying the reversible coding system for pictures to a coding algorithm of the MPEG-2 (Moving Picture Experts Group-Phase 2).
MPEG-2 coding originally is non-reversible coding. However, MPEG-2 coding is non-reversible only at the quantization part thereof. The entire coding can be made reversible coding by replacing the discrete cosine transform and quantization parts with reversible discrete cosine transform. In the prior art by the inventor, a reversible moving picture coding system is implemented by the countermeasure just described.
FIG. 7 shows in block diagram a construction of an encoder which effects reversible moving picture coding.
Referring to FIG. 7, a motion estimation circuit 5 effects motion estimation between a picture coded in the past and a picture of an object of coding and outputs a motion vector for each macro block. A predictor 4 produces a motion compensated predictive picture from the reference picture in accordance with motion vectors outputted from the motion estimation circuit 5 and outputs the motion compensated predictive picture. An adder 3 subtracts the motion compensated predictive picture outputted from the predictor 4 from the coding object picture to produce a predictive error picture. A reversible discrete cosine transform (lossless DCT; LDCT) circuit 1 performs reversible discrete cosine transform for the predictive error picture outputted from the adder 3 for each block and outputs transform coefficients.
It is to be noted that the encoder shown in FIG. 7 does not include a local decoding loop which is included in an ordinary MPEG-2 encoder. This is because, in reversible coding, a locally decoded picture coincides completely with the original picture and no local decoding is required.
In the MPEG-2, an object of coding is an interlaced scanned moving picture. When discrete cosine transform is performed, either field scanned blocks, that is, blocks obtained by interlaced scanning, or frame-scanned blocks, that is, progressively scanned blocks, can be selected. As a selection controlling method for such selection, for example, a method of the MPEG-2 TM-5 (Test Model 5) or another method disclosed in Japanese Patent Laid-Open Application No. Heisei 8-102950 (title of the invention: Coding Apparatus and Motion Discrimination Method) is known.
In the MPEG-2 TM-5, a correlation coefficient of pixel values is determined for each macro block between fields to effect selection of frame-scanning or field-scanning. The magnitude of the correlation coefficient is discriminated with a threshold value, and if the correlation coefficient is higher than the threshold value, then frame-scanning is selected, but in any other case, field-scanning is selected to effect discrete cosine transform.
In the coding apparatus disclosed in Japanese Patent Laid-Open Application No. Heisei 8-102950 mentioned above, a difference in pixel value is determined for each block between fields, and selection between frame discrete cosine transform and field discrete cosine transform is controlled based on an absolute value of a sum when such differences are added in a vertical direction. More particularly, an absolute value of a difference sum described above is determined for each column first. Then, the absolute values of the difference sums of the columns are added to determine a sum total, and the sum total is compared with the threshold value. Simultaneously, a maximum value among the absolute values of the difference sums of the columns is determined, and the maximum value is compared with the threshold value. Then, if both of the sum total and the maximum value are higher than the threshold value, then field discrete cosine transform is selected, but in any other case, frame discrete cosine transform is selected.
The selection control described above can be incorporated into the system shown in FIG. 7. In particular, frame discrete cosine transform and field discrete cosine transform can be adaptively changed over using the prior art by the inventor. The applied system is shown in FIG. 8.
Referring to FIG. 8, a frame/field adaptive-scanned block production circuit 6 adaptively selects field scanning or frame scanning based on a conventional system from a predictive error picture outputted from the adder 3 to produce a block and outputs the thus produced block.
However, even if the system of the MPEG-2 TM-5 or the system disclosed in Japanese Patent Laid-Open Application No. Heisei 8-120950 mentioned hereinabove is combined with the prior art by the inventor, a desired result cannot necessarily be obtained from the point of view of compatibility between a reversible discrete cosine transform coding system and a conventional discrete cosine transform coding system. Here, the compatibility signifies a matter of which degree of picture quality is obtained when a picture coded using reversible discrete cosine transform is decoded by ordinary inverse discrete cosine transform. It is considered that the higher the picture quality obtained then, the superior in compatibility the reversible discrete cosine transform coding system.
The reason why a desirable result cannot be obtained is that the conventional adaptive selection system between frame discrete cosine transform and field discrete cosine transform is optimized in terms of reduction of the code amount where ordinary discrete cosine transform is used, and does not take augmentation in compatibility into consideration. Conversely speaking, the selection system can be re-examined from the point of view of augmentation of the compatibility.